The present invention relates to creating discontinuities in the upper portion of concrete slabs, also known as contraction joints. In particular, the present invention provides for inserting a flexible material such as tape into the upper portion of a plastic concrete slab to create a contraction joint.
Contraction joints serve the purpose of controlling the location of cracks in concrete slabs due to thermal and drying shrinkage behavior of concrete. The joints weaken the slab so cracks occur at the joint locations, and in, this manner, minimize the occurrence of random cracks. Conventional knowledge proposes that the depth of contraction joints should be one-fourth the thickness of the slab, and spacing of the joints to range from 24 to 36 times the thickness of the slab.
Several methods of installing contraction joints are currently in use by the construction industry. The oldest method uses a grooving tool to create a score line or joint in a slab. Typically grooving tools are employed for small slabs such as sidewalks. Inserting strips of wood, plastic, or metal to function as contraction joints is also standard, but problems occur when the strips are not installed vertically, which happens when aggregate particles obstruct insertion. Also, strips that are not pushed flush or below the surface can interfere with finishing the surface of the concrete. The method of inserting materials for contraction joints is usually limited to medium sized slabs, such as residential house slabs and small warehouses.
Today, the most common method of installing contraction joints is by saw cutting. The original saw cutting method used water-cooled saw blades, where it was necessary to allow the concrete to develop sufficient strength, prior to saw cutting, in order for aggregate particles to resist raveling along the edges of the saw cut. A time delay was required before saw cutting to allow concrete to develop strength, and in many cases, this delay permitted random cracks to develop before the contraction joints were installed. The newer saw cutting method, called early entry saw cutting (U.S. Pat. No. 5,582,899 “Concrete Surface with Early Cut Grooves” by Chiuminatta, Dec. 10, 1996) uses a machine that allows cutting of the concrete immediately after final finishing operations. Cutting concrete at this early age has successfully minimized random cracks because the joints were installed before thermal contraction or drying shrinkage of concrete occurred. Limitations with the method are the slow process of saw cutting concrete and the high expense of diamond blades. The installed cost of contraction joints made according to the present invention is less than half that of using the method of early entry saw cutting.
A common problem exists with many of the prior art methods of creating contraction joints. Cracks that occur at contraction joint locations are paths for moisture from below the slab to the surface. If floor covering, such as sheet vinyl, carpet, or wood, are used, then a problem can occur by the presence of moisture under the slab. One common problem in the house building industry is related to vinyl floor coverings bonded to concrete slabs by latex adhesive. At contraction joint locations, where moisture from the sand or soil below the slab can easily rise up through the crack in the slab, an environment is created for mold growth on the underside of the vinyl. The same situation can occur with carpet and wood, but wood is also vulnerable to expansion and warping by absorbing moisture. To prevent moisture from damaging the floor coverings, contraction joints need to be filled, typically with epoxy or polyurethane materials. This operation is time consuming and expensive. The current invention eliminates the expense of filling contraction joints.
Prior art methods of inserting flexible materials into fresh concrete are reviewed herein. The most relevant patents were by J. N. Heltzel. The first patent was U.S. Pat. No. 1,697,563, “Concrete Surfacing Joint and Means for and Method of Forming the Same”, issued Jan. 1, 1929. The method used a mandrel to push into plastic concrete a flexible material to form a contraction joint. Discrete lengths of strip material are laid out on the surface of the concrete slab and then a mandrel pushes the strip of material into the concrete using vibration or jarring action. This is a step-by-step process of inserting discrete lengths of material into the concrete. The method required “water-proof paper, fabric, rubber, sheet metal, or other easily bendable material,” but all the materials had to be water-proof because the material was laid out on the fresh concrete prior to being pushed into the concrete; and, if non-waterproof paper were used, the paper would absorb water and be too weak to resist the action of insertion. The current invention overcomes these limitations by using a rotating insertion blade to fold the material immediately prior to insertion into the concrete; hence, non-waterproof paper can be used and a continuous operation can occur by using an “endless” strip of material that is supplied by a spool carried on the device.
The second patent by Heltzel was U.S. Pat. No. 1,740,345, “Traffic Line and Joint Machine and Method of Producing Same”, issued Dec. 17, 1929. This invention describes a large machine that spans highway pavement for inserting long strips of material into the pavement. The machine can install longitudinal and transverse joints. A rotating blade is used to form a slot in the plastic concrete followed by separate operations where a strip of material is laid out over the slot and then a long mandrel blade is used to push the material into the concrete. The flexible material is mounted on a roll. In contrast, the current invention continuously installs a strip of material into concrete. The rotating insertion blade pushes the tape into the concrete in one operation, which eliminates the separate operations, required by Heltzel's device, of creating a slot in the concrete and then laying out the strip of material, and then inserting the material into the concrete.
The third patent by Heltzel, U.S. Pat. No. 1,997,216, “Method of and Means for Producing Concrete Surfacing Joints”, issued Apr. 9, 1935, provided for manually cutting a slot in plastic concrete by using a rotating blade. After cutting the slot, in a separate operation material is laid over the slot, and then the rotating blade is used to insert the material into the concrete. Three operations were required. The current invention uses one operation to insert an “endless” strip of material into concrete for the contraction joint.
Later art by Gunert, U.S. Pat. No. 3,194,130, “Apparatus for Forming a Weakened Zone in Pavements”, issued Jul. 13, 1965, describes a large machine that continuously creates a slot in plastic concrete and inserts material fed off of a roll. The insertion material is stiff in its vertical orientation and flexible in its horizontal orientation. The slot is formed by two blades, which are spaced such that the insertion material is run between these blades in its vertical orientation as it is embedded into the concrete. The material is not folded, as is the case for the current invention. The machine is used only for forming contraction joints in the longitudinal direction of pavements. No method was described to form joints transverse to the longitudinal joints.
L. A. Shaw and R. D. Shaw received patents that use a folded strip of material for contraction joints, U.S. Pat. Nos. 7,334,962 and 7,381,007, issued Feb. 26, 2008 and Jun. 3, 2008, respectively. Their method installs taut wires in a form over which is hung flexible material. This operation occurs prior to casting concrete. The folded material forms the contraction joint. These patents do not cover the current invention, which inserts material into concrete after placement.